The present invention concerns the processing of semiconductor devices and pertains particularly to the removal of silicon oxynitride material using a wet chemical process after gate etch processing.
The gate patterning step is arguably the most critical step in the process flow for integrated circuits. To improve the photolithography patterning capability, a 270 Angstrom (xc3x85) inorganic bottom anti-reflective coating (BARC) of silicon oxynitride (SiON) can be deposited beforehand. However, this SiON layer must be removed from atop the gate stack before a metal silicide can be formed. Any SiON remaining greatly hinders the uniform formation of a low sheet resistance silicide.
In the prior art, the SiON layer is removed as a part of the spacer etch. The spacer etch process etches a blanket-deposited dielectric film to form a spacer with which to separate the lightly doped drain (LDD) and source/drain regions. After the spacer is formed, an overetch is performed to remove the SiON layer. This overetch is typically performed in a dielectric etcher with gases such as CF4, CHF3, and Ar. However, this overetch results in several undesired effects: loss of silicon trench isolation oxide, undesired etching of the diffusion silicon, and recessing of the spacer.
In accordance with the preferred embodiment of the present invention, a method is presented for forming a transistor gate structure. A gate oxide layer is formed. Gate material is deposited on the gate oxide layer. A layer of silicon oxynitride is deposited on the gate material. The layer of silicon oxynitride, the gate material and the gate oxide layer are etched to form a gate structure. A silicon oxynitride layer remains on top of the gate structure. A wet chemical process is performed to remove the silicon oxynitride layer from the top of the gate structure. After performing the wet chemical process, conventional spacers are formed around the gate structure, without the need for an overetch.
In the preferred embodiment of the present invention, the gate material is amorphous silicon. Alternatively, the gate material can be polysilicon, annealed amorphous silicon or another type of gate material. The wet chemical process is, for example, a hot phosphoric acid etch or a neutral oxide etch (NOE). NOE is a solution containing ethylene glycol, ammonium fluoride, water and surfactant. The wet chemical process may be performed before or after a lightly doped drain is implanted.
Selectively removing the gatestack SiON layer by a wet strip process after gate etch (instead of as a part of spacer overetch) reduces trench oxide loss, etching of the diffusion silicon, and recess of the spacer. The wet strip process selectively etches the SiON layer, but leaves the remainder of the wafer, most importantly the gate oxide (e.g., a thermal oxide), untouched. With the SiON layer removed, the spacer dielectric is deposited directly onto the gate material. Then, with the spacer etch, the SiON overetch step can be removed, eliminating the associated undesired effects.